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A dictionary in Python is a collection of key-value pairs. It is unordered, mutable, and does not allow duplicate keys. Dictionaries are useful for storing data in key-value format, making lookups fast and efficient.
1. Creating a Dictionary
You can create a dictionary using curly braces {} or the dict() function.
# Creating a dictionary using curly braces
student = {
"name": "John",
"age": 22,
"course": "Computer Science"
}
print(student)
# Creating a dictionary using dict()
person = dict(name="Bob", age=25, city="New York")
print(person)
✅ Key Points:
- Each key must be unique.
- Values can be of any data type (int, string, list, another dictionary, etc.).
- Keys must be immutable (strings, numbers, or tuples but not lists).
2. Accessing Dictionary Elements
You can access values using square brackets [] or the .get() method.
student = {"name": "John", "age": 22, "course": "Computer Science"}
# Accessing using key
print(student["name"]) # Output: John
# Using .get() method (avoids error if key is missing)
print(student.get("age")) # Output: 22
print(student.get("city", "Not Found")) # Output: Not Found
✅ Why use .get()?
- If a key does not exist,
student["city"]gives an error, butstudent.get("city")returnsNone(or a default value).
3. Adding & Updating Dictionary Items
You can add or modify dictionary values by assigning a value to a key.
# Adding a new key-value pair
student["city"] = "New York"
print(student) # {'name': 'John', 'age': 22, 'course': 'Computer Science', 'city': 'New York'}
# Updating an existing key
student["age"] = 23
print(student) # {'name': 'John', 'age': 23, 'course': 'Computer Science', 'city': 'New York'}
4. Removing Elements from a Dictionary
You can remove key-value pairs using del, pop(), or popitem().
student = {"name": "John", "age": 22, "course": "Computer Science"}
# Using del (removes key permanently)
del student["age"]
print(student) # {'name': 'John', 'course': 'Computer Science'}
# Using pop() (removes and returns the value)
course = student.pop("course")
print(course) # Output: Computer Science
print(student) # {'name': 'John'}
# Using popitem() (removes the last inserted item)
student["city"] = "New York"
student.popitem()
print(student) # {'name': 'John'}
✅ Difference between pop() and del:
pop("key")returns the removed value.del dictionary["key"]deletes without returning anything.
5. Dictionary Methods
Here are some useful dictionary methods:
Method Description
dict.keys() Returns all keys
dict.values() Returns all values
dict.items() Returns key-value pairs as tuples
dict.update(new_dict) Merges two dictionaries
dict.pop("key") Removes a key and returns its value
dict.popitem() Removes and returns the last key-value pair
dict.clear() Removes all elements
Example:
student = {"name": "John", "age": 22, "course": "Computer Science"}
# Getting all keys
print(student.keys()) # dict_keys(['name', 'age', 'course'])
# Getting all values
print(student.values()) # dict_values(['John', 22, 'Computer Science'])
# Getting all key-value pairs
print(student.items()) # dict_items([('name', 'John'), ('age', 22), ('course', 'Computer Science')])
6. Iterating Through a Dictionary
You can loop through a dictionary to access keys, values, or both.
student = {"name": "John", "age": 22, "course": "Computer Science"}
# Loop through keys
for key in student:
print(key) # name, age, course
# Loop through values
for value in student.values():
print(value) # John, 22, Computer Science
# Loop through key-value pairs
for key, value in student.items():
print(f"{key}: {value}")
✅ Best practice: Use .items() to access both keys and values in a loop.
7. Merging Dictionaries (update())
You can combine two dictionaries using the .update() method.
dict1 = {"name": "John", "age": 22}
dict2 = {"city": "New York", "age": 23} # Overwrites 'age' key
dict1.update(dict2)
print(dict1) # {'name': 'John', 'age': 23, 'city': 'New York'}
✅ If a key exists in both dictionaries, .update() overwrites it.
8. Dictionary Comprehension
You can create dictionaries dynamically using dictionary comprehension.
squares = {x: x*x for x in range(1, 6)}
print(squares) # {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
✅ Useful when creating dictionaries dynamically.
9. Nested Dictionaries
A dictionary can have another dictionary as its value.
students = {
"Alice": {"age": 22, "course": "CS"},
"Bob": {"age": 23, "course": "Math"}
}
print(students["John"]["course"]) # Output: CS
✅ Used for storing structured data (like JSON objec